ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-6-2147-2006Effect of humidity on nitric acid uptake to mineral dust aerosol particlesVlasenkoA.13SjogrenS.2WeingartnerE.2StemmlerK.1GäggelerH. W.13AmmannM.11Laboratory of Radio- and Environmental chemistry, Paul Scherrer Institute, Villigen, Switzerland2Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland3Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland200620066821472160This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This article is available from http://www.atmos-chem-phys.net/6/2147/2006/acp-6-2147-2006.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/6/2147/2006/acp-6-2147-2006.pdf

This study presents the first laboratory observation of HNO<sub>3</sub> uptake by
airborne mineral dust particles. The model aerosols were generated by dry
dispersion of Arizona Test Dust (ATD), SiO<sub>2</sub>, and by nebulizing a
saturated solution of calcium carbonate. The uptake of <sup>13</sup>N-labeled
gaseous nitric acid was observed in a flow reactor on the 0.2&ndash;2 s reaction
time scale at room temperature and atmospheric pressure. The amount of
nitric acid appearing in the aerosol phase at the end of the flow tube was found to be a linear function of the aerosol surface
area. SiO<sub>2</sub> particles did not show any significant uptake, while the
CaCO<sub>3</sub> aerosol was found to be more reactive than ATD. Due to the
smaller uncertainty associated with the reactive surface area in the case of
suspended particles as compared to bulk powder samples, we believe that we
provide an improved estimate of the rate of uptake of HNO<sub>3</sub> to mineral
dust. The fact that the rate of uptake was smaller at a concentration of 10<sup>12</sup>
than at 10<sup>11</sup> was indicative of a complex uptake mechanism. The uptake coefficient averaged over the first 2 s of reaction time at
a concentration of 10<sup>12</sup> molecules cm<sup>-3</sup> was found to increase with
increasing relative humidity, from 0.022&plusmn;0.007 at 12% <i>RH</i> to
0.113&plusmn;0.017 at 73% <i>RH</i> , which was attributed to an increasing degree of solvation of the more basic minerals.
The extended processing of the dust by higher concentrations of HNO<sub>3</sub> at 85% <i>RH</i> led to a water soluble coating on
the particles and enhanced their hygroscopicity.